© Copyright 1997, The University of New Mexico 4-1 LAN Technologies Ethernet Token Ring FDDI...

4-1© Copyright 1997, The University of New Mexico

LAN Technologies

• Ethernet• Token Ring• FDDI• Gigabit Ethernet


IEEE/ISO Protocol Set

• Deal with Physical and Data Link layers only• 3 medium access control (MAC) standards

– IEEE 802.3 CSMA/CD; ISO 8802.3

– IEEE 802.4 Token bus; ISO 8802.4

– IEEE 802.5 Token ring; ISO 8802.5

Physical

Logical Link Control (LLC)

Medium Access Control (MAC)

Layer 1

Layer 2


Ethernet

• Developed in early 1970’s Xerox PARC• Standardized as IEEE 802.3• Provides data communications for LAN systems• Baseband technology developed by Xerox, Intel, Dec• Broadband version developed by Mitre• Frame based transmission• Most commonly used medium access control


Ethernet

• CSMA/CD– Carrier Sense Multiple Access with Collision Detection

• Nodes listen before transmitting• Nodes may transmit at any time• Collisions result in back-off and retransmission

– random access contention based media access• stations contend for access to the medium


CSMA/CD

Bus

A B C

A B C

A B C

Collision

If too many collisions or other errors(CRC alignment, Jabber, Giant frames)occur on a given port, hubs will auto partition that port to prevent it from repeating on the entire segment.


Ethernet

• MAC frame: 64 bytes min

Preamble SDF DA SA Length LLC data Pad FCS

7 1 2 or 6 2 or 6 >/ 02 >/ 0 4 bytes

Preamble SDF DA SA Type LLC data Pad FCS

7 1 2 or 6 2 or 6 >/ 02 >/ 0 4 bytes

IEEE 802.3

Ethernet


Ethernet

• IEEE 803.2 MAC frame– Preamble: 7 byte pattern of alternating 0s and 1s used by the receiver

to establish bit synchronization– Start frame delimiter (SDF): sequence 10101011 indicating start of

frame, enables receiver to establish first bit of frame– Destination address (DA): specifies the stations for which the frame

is intended ( 2 or 6 bytes, implementation decision, usually 6 bytes)• it may be one of the following

– a unique physical address– a group address– a global address


Ethernet

• Source address (SA): specifies the station that sent the frame (2 or 6 bytes, usually 6 bytes)

• Length: length of the LLC data field• LLC data: data unit supplied by LLC, less than 1518

bytes• Pad: octets added to ensure the frame is long enough

for the proper collision detection operation (optional)• Frame check sequence (FCS): 32-bit cyclic redundancy

check (CRC); except preamble, SDF and FCS


Ethernet

• Ethernet v2– Type: deals with protocols

• IP protocol, Type = 0800

• No two machines should have the same physical address

• LSB of first octet – if 0 then unique address

– if 1 then multicast address


Ethernet

• 10Mbps transmission– Manchester Encoding

• Baseband 20MHz• Network Interface Cards

– Each node has a NIC

– NIC is responsible for transmitting & receiving data frames

– Each NIC has a different address• MAC address


Ethernet- Network Interface Card

82503PLX9032or

PLX9036

1 MegFlash

82556 BIG MAC100 MbpsThunderModule

16 KSRAMFIFO

RJ45RelayMags

Mags


MAC Address

• Media Access Control Address• OSI Layer 2• 6 Bytes

– 1st 3 Bytes- Manufacturer ID

– 2nd 3 Bytes- Random

• 00-00-81-10-a3-4b– Vendor Specific 10-a3-4b


Ethernet

• Thick Coax (Thicknet)– IEEE 10Base-5

– Maximum distance of 500m

– Nodes “tap” into cable with transceivers

– Supports up to 250 nodes per cable segment

– Must be terminated at both ends

– Bus topology

– 50 ohm coaxial cable


Ethernet- Thick Coax

TAP TAP TAPTAP

XCVR XCVRXCVRXCVR

AUI Cable

Thick Coax


Ethernet

• Thin Coax (Thinnet)– IEEE 10Base-2

– Maximum distance 180m

– Supports up to 30 nodes per cable segment

– Nodes connect via BNC connectors

– Must be terminated at both ends

– Bus topology

– 50 ohm coaxial cable


Ethernet- Thinwire

.

Thin Coax

BNC BNC BNC BNC

BN

C

BN

C

BN

C

BN

C


Ethernet

• Unshielded Twisted Pair (UTP)– IEEE 10Base-T

– Star topology from hub

– EIA/TIA category 3, 4, 5 supported

– Uses RJ-45 connectors

– Maximum distance 100m (500 m with fiber)

– Supports 1 node per cable segment


Ethernet- UTP

Hub

UTP Cable


10BaseT Pinout

RJ45 Connector

1 2 3 4 5 6 7 8

TX+ TX- RX+ RX-

Note that TX and RX are going to be reversed on the hub side,Thus when connecting hub to hub,the need for MDI/MDIX. MDIX looks for a NIC, MDI looks for a repeater port


Ethernet

• 10BROAD36– 75 ohm coaxial cable, CTV

– Bus/tree topology

– Maximum segment length is 1800 m

– Signaling technique is broadband (DPSK)


Ethernet

• Fiber Optic– IEEE 10Base-F, 10Base-FL, 10Base-FOIRL

– Star topology from hub

– Multi Mode Fiber supported

– MMF supports up to 2km distance

– Point-to-Point

– Secure


Fast Ethernet

• Set of specifications developed by IEEE 802.3 committee to provide Ethernet compatible LAN operating at 100Mbps

• Specifications are:– 100Base-TX: 2 CAT 5 UTP

– 100Base-FX: 2 optical fiber

– 100Base-T4: 4 CAT 3 or CAT 5 UTP


Ethernet- Repeaters

Repeater

500m 500m

500 meters between repeaters for 10Base5 - Thicknet

2 repeater limit between any two stations any more and you need to insert a bridge/switch or a router


Ethernet- Equipment

• Hubs– Commonly referred to as concentrators or multiport

repeaters

– Used to connect multiple cable segments

– Retimes & Regenerates signal

– May support multiple cable types


Ethernet- Hubs

Hub

UTP Cable

If too many collisions or other errors(CRC alignment, Jabber, Giant frames)occur on a given port, many Bay Networks hubs will auto partition that port to prevent it from repeating on the entire segment.


Token Ring Logical Operation

• Control token concept by IBM and GM

• Deterministic access method, no contention

• Two main standards: IEEE 802.5 MAC protocol, FDDI

• Where Ethernet uses CSMA/CD, Token Ring uses a token passing mechanism where one station is allowed to talk at a time

• Traffic moves in a logical circle, going from station to station, until the appropriate station receives it and holds on to it


Token Ring Physical Operation

• Ring is emulated through TR hub by passing frames from station to station (1, 4 or 16Mbps).

• When a station has a problem or is pulled from the ring, that port is “wrapped” to allow the ring to continue

• An active hub retimes and regenerates the signal, a passive hub connects the ring

• When a station inserts at the wrong speed or loses track of its neighbor, a beacon condition is created on the ring


Token Ring Pros and Cons

• Light load: TR is not very efficient. Stations must wait for the token to arrive

• Heavy load: TR operates in a round robin fashion• Priority and reservation schemes can be implemented• Bandwidth can be guaranteed• High overhead for token maintenance• Loss of token prevents further use or TR• Duplication of token disrupts operation• A monitor station is need to manage the TR network


FDDI

• Defined by four separate specifications

Logical LinkControl

Media AccessControl (MAC)

Physical LayerControl (PHY)

Physical Layer Medium (PMD)

StationManagement(SMT)

Fiber DistributedData Interface (FDDI)Standards

Layer 2

Layer 1


FDDI

• MAC: how medium is accessed, frame format, token handling, addressing, CRC check, error recovery

• PHY: defines data encoding/decoding, clocking requirements, and framing

• PMD: defines characteristics of transmission medium, power levels, bit error rates, optical components, connectors


FDDI

• SMT: defines FDDI station configuration, ring configuration, ring control features, station insertion and removal, initialization, fault isolation and recovery, scheduling, and collection of statistics


FDDI

• Follows the token ring scheme• Similar to IEEE 802.5• Has no priority or reservation bits• Designed for LANs and MANs• Accommodates higher data rates of 100Mbps


FDDI

• FDDI operationPrimary Ring

Secondary RingSingle Attach Station

Dual Attach Station


Primary and Secondary Rings

• Primary and Secondary Rings– normal traffic is on primary ring

– secondary “counter-rotating” ring is redundant - for primary failure

• Three Paths– primary - normal conditions

– secondary - backup

– local - intelligent insertion and removal


FDDI Stations

• Dual Attached Stations (DAS)– connected to both rings

• Single Attached Stations (SAS)– attached only to the primary ring through

a hub/concentrator port

SAS SASSAS

ConcentratorDASPrimary RingSecondary Ring


FDDI Token Frame

• Preamble: 64 bits• Starting Delimiter (SD): 8 bits• Frame Control (FC): 8 bits

– bit format of 10000000 or 11000000 to indicate that it is a token

• Ending Delimiter (ED): 4 bits– contains a pair of non-data symbols (T) that terminate

token framePreamble SD FC ED


FDDI Frame

• Preamble: 64 bits– synchronize frame with each station’s clock

– repeating stations may change length of the field as per clocking requirements

• Starting Delimiter (SD): 8 bits– indicates beginning of a frame;

– coded as JK where J and K are non-data symbols; J same polarity as preceding symbol, K opposite polarityPreamble SD FC EDDA SA DATA FCS FS


FDDI Frame

• Frame Control (FC): 8 bits– has bit format CLFFZZZZ

– C indicates synchronous or asynchronous frame

– L indicates use of 16 or 48 bit addresses

– FF indicates whether it is a LLC, MAC control or reserved frame

– in a control frame ZZZZ indicates the type of control

• Destination Address (DA): 16 or 48 bits– specifies station for which the frame is intended

• Source Address (SA): 16 or 48 bits– specifies station that sent the frame


FDDI Frame

• Data: 0 - 5000 bytes– contains an LLC data unit or information related to a control operation

• Frame Check Sequence (FCS): 32 bits– cyclic redundancy check based on FC, DA, SA and Data fields

• Ending Delimiter (ED): 4 bits– contains a non-data symbol T; T=0 token frame, T=1 normal frame

• Frame Status (FS): 1 bit– contains one of the following:

• E: error detected• A: address recognized• F: frame copied


FDDI Limitations

• STP, UTP, Single and Multimode Fiber• implementation over copper is known as CDDI• 500 stations per ring• 200 km or 124 miles total length• 10 km between station and concentrator single mode• 2 km between station and concentrator multimode


FDDI Ports

• DAS A - Primary In, Secondary Out• DAS B - Primary Out, Secondary In• Master - concentrator port only• Slave - SAS only primary in and out

Primary

Secondary

PortB

Port A

FDDI DAS


Optical Bypass Switch

• When a station is off this goes into by-pass mode• Prevents Ring-Wrap

A

B

MAC

Station

Ring Wrap


FDDI Architecture

• Use only concentrators on the FDDI dual ring– Minimize chance of ring segmentation by reducing number of

components on dual ring

– Enhance scalability by allowing stations to be added and removed without disrupting the dual ring

– Do not use ring wiring unless forced to do so by existing cable plant

• Place all equipment attached to the FDDI dual-ring in one or two physically secure locations


FDDI Architecture

• Dual home critical bridges, routers and servers – provide physical layer redundancy– Connect remote locations to the FDDI backbone via dual-

homed routers and/or concentrators

• Single attach non-critical desktop connections

• Use star wiring wherever possible


What is Gigabit Ethernet?

Speed.......................................Cost.........................................

IEEE Standard......................Media Access Protocol..........Frame Format........................

Topology.................................Cable support.........................Network diameter (max).......UTP link distance (max)........Media independent interface

Full duplex capable?.............

Broad multivendor support..Multivendor Availability.......

10 MbpsX

802.3CSMA/CDIEEE 802.3

Bus or starCoax, UTP, fiber2,500 meters100 metersYes (AUI)

Yes

YesNow

100 Mbps2X 10BT

802.3uCSMA/CDIEEE 802.3

StarUTP, fiber210 meters100 metersYes (MII)

Yes

YesNow

Ethernet Fast Ethernet

1000 Mbps2-4X 100BT

802.3zCSMA/CDIEEE 802.3

StarUTP, fiber200 meters100 metersYes (G-MII)

Yes

Yes1998

Gigabit Ethernet


Gigabit Ethernet Technology

10 Mbps Ethernet

CSMA/CD MAC

Thick Coax(10Base5)

Thin Coax(10Base2)

Fiber(10Base-

F)

Twisted Pair

(10Base-T)

100 Mbps Ethernet

CSMA/CD MAC

Four Pair UTP(100Base-T4)

Fiber(100Base-FX)

Two Pair UTP, STP

(100Base-TX)

No change

AUI MII

1000 Mbps Ethernet

CSMA/CD MAC

Four Pair UTP(TBD)

MM FiberCD Laser

1300nm Laser

SM Fiber1300nm Laser

G-MII

Minor change


Gigabit Ethernet Topology

• CSMA/CD Topology– Supports 100 Meter distance for CSMA/CD protocol

over all media options.

• Full Duplex Topology– Supports variable distance dependent on media

capabilities

– Supports server, switch and router DTE applications.


Gigabit Ethernet vs. ATM

Link Speed.............................

Standards.. .............. ..............Shared Access Protocol.........Switched Access Protocol.....Frame Format........................Transmission Control............Redundancy............................VLAN support........................Network diameter (max).......UTP link distance (max)........

Priority...................................Full duplex switching............

Broad multivendor support..Multivendor Availability.......

10, 100, 1000 Mb/s

IEEE 802.3CSMA/CDN/AIEEE 802.3 FrameConnectionless802.1 Spanning Tree802.1Q (?)200 meters100 meters

802.1P (?)Yes

Yes1998

Ethernet(Gigabit)

25, 155, 622, 1.2 Gb/s

ATM ForumN/ASVC/CMSATM CellConnection OrientedATM MeshLANE2,500 meters100 meters

ATM QoSYes

YesNow

ATM


Three generations of LANs

• Apple, Bellcore, Sun and Xerox identified the three generations of LANs in their document [ABSX92].


First Generation LANs

• Typified by CSMA/CD and Token ring LANs.• Provided terminal to host connectivity at moderate

data rates.• Supported client server architecture at moderate data

rates.


Second Generation LANs

• Typified by FDDI.• Supported backbone LANs.• Supported high performance work stations.


Third Generation LANs

• Typified by ATM LANs.• Provide aggregate throughputs and real time

transport guarantees for multimedia applications.


Further Criteria For Third Generation LANs

• Support multiple, guaranteed classes of service.– examples:

• a live video application requires 2 Mbps guaranteed.• a file transfer uses background class of service.

• Provide scaleable throughput in terms of per host capacity.– enabling applications requiring large volumes of data in and out of a

single host.

• Provide scaleable throughput in terms of aggregate capacity.– enabling installations to grow from a few to several hundred high

performance hosts.

• Internetwork between LAN and WAN technology.


ATM - A Third Generation LAN

• Multiple classes of service provided by virtual paths and virtual channels.

• Permanent connections provide pre-configured service.

• Switched connections provide on demand service.

• ATM is scaleable– more switching nodes can be added.

– higher data rates can be used for attached devices.

• Cell based data transport at high rates allow wide area networking and local area networking, with seamless connectivity between the two.


Types of ATM LANs

• Gateway to ATM WAN– ATM switch acts as a router and traffic concentrator,

linking premises networks to an ATM WAN.

• Backbone ATM switch– Single ATM switch interconnects LANs.

– Network of ATM switches interconnects LANs.

• Workgroup ATM– End systems(high performance multimedia workstations)

connect directly to an ATM switch.


Data Communications With Processors

• Two types– Input/Output channel

– Network communications


I/O Channel Features

• Direct point to point or multipoint high speed communications link over very short distances.

• Hardware based link.

• Data is transferred between a buffer at source device and a buffer at destination device.

• No special formatting is used, data is transferred in raw form.

• Transfer control and error detection is implemented in hardware.

• I/O channel links processors with peripheral devices.


Network Communications Features

• Collection of interconnected access points.• Software protocol structure enables communication.• Many different types of data transfer are allowed.• Network protocols that provide flow control, error

detection and error recovery are implemented in software.

• Data transfer, between end systems in the local, metropolitan and wide area distances, is managed.


Fiber Channel Features

• Has simplicity and speed of channel communications.• Implements communications oriented protocols as in network

communications.• Provides interconnectivity between end systems like in the

network communications and also short distance connectivity as in the case of channel communications.

• One multiprotocol interface is used to provide:– Peripheral connectivity.– Host to host internetworking.– Connected processor clusters.– Multimedia capabilities.


Channel Oriented Features of Fiber Channel Architecture

• Data-type qualifiers for routing frame payload into particular interface buffers.

• Link-level constructs associated with individual I/O operations.

• Protocol interface specifications to allow support of existing I/O channel architectures, such as the small computer interface (SCSI).


Network Oriented Features of Fiber Channel Architecture

• Full multiplexing of traffic between multiple destinations.

• Peer-to-peer connectivity between any pair of ports on a Fiber channel network.

• Capabilities of internetworking with other technologies.


Fiber Channel Requirements as per Fiber Channel Association

• Full duplex links with two fibers per link.• Performance from 100 Mbps to 800 Mbps on a

single link(200 Mbps to 1600 Mbps per link).• Support for distances upto 10 kms.• Small concentrators.• High capacity utilization with distance insensitivity.


Fiber Channel Requirements as per Fiber Channel Association

• Greater connectivity than existing multidrop channels.

• Broad availability.• Support for multiple cost/performance levels, from

small systems to super-computers.• Ability to carry multiple existing interface command

sets for existing channel and network protocols.


Fiber Channel Elements (Slide 1)

• Nodes

– These are the end systems

• Fabric

– Collection of switching elements

– Fabric connects nodes and switches by point to point links

– Routes frames between nodes connected to the fabric

– May buffer frames to enable connections at different data rates

– Can be a single fabric element or can be a collection of fabric elements networked together


Fiber Channel Elements (Slide 2)

• N_port– Port on a node which participates in a point to point link.

• Frames– Data units used for communications on the point to point

links.

• F_port– Port on a switching fabric which participates in a point to

point link.


Fiber Channel Vs. Other LANs

• Circuit switched or packet switched as opposed to shared medium LAN.

• No medium access control issues.• Scales easily in :

– Number of N_ports– Data rate– Distance covered

• New transmission media, technology and data rates can be accommodated by adding new switches and F_ports to existing fabrics.


Fiber Channel Protocol Architecture (Slide 1)

• FC-O physical media– Optical cable with laser or LED transmitters for long

distance transmissions.

– Copper coaxial cable for highest speeds over short distances.

– Shielded twisted pair for low speeds over short distances.

– Rates of 100 Mbps, 200Mbps, 400Mbps, 800Mbps


Fiber Channel Protocol Architecture (Slide 2)

• FC-1: Transmission protocol– Byte synchronization and encoding

– 8B/10B encoding/decoding scheme • Balanced scheme

• Simple to implement

• Has useful error detection capability

– Special code character maintains byte and word alignment


Fiber Channel Protocol Architecture (slide 3)

• FC-2: Signaling protocol– Transport mechanism

– Framing protocol and flow control between N_ports

– Three classes of service between ports

• FC-3: Common Services– Port related services

– Services that appear across two or more ports in a node


Fiber Channel Protocol Architecture (slide 4)

• FC-4: Upper layer protocols– Supports a variety of channel and network protocols.

– Channel:• SCSI, HIPPI,IPI-3, SBCS


Fiber Channel Physical Interface and Media

• Specified by fiber channel FC-0• Several data rate allowed :

– 100 Mbps to 800 Mbps

• Several mediums allowed :– Optical fiber– Coaxial cable– Shielded twisted pair

• Maximum distances for point-to-point links range from 50 meters to 10 Km.


Fiber Channel Transmission Protocol

• Specified by fiber channel FC-1• Defines:

– Signal encoding technique for transmission• 8B/10B encoding scheme is used.

• 8 bits of data from level FC-2 is converted into 10 bits for transmission.

– Signal encoding technique for synchronization


Fiber Channel Common Services (Slide 1)

• Specified by fiber channel FC-3.• Striping

– Transmit a single information unit across multiple links simultaneously.

– Good for transferring large data sets in real time.

– Good for video-imaging applications.



• Hunt groups– Set of associated N_ports at a single node identified by an

alias.

– Any frame sent to the alias is processed by an available N_port within the set.

– Good for decreasing latency.



• Multicast– Sends to all N_ports on a fabric.

– Sends to a subset of the N_ports on a fabric.


Fiber Channel Mapping

• Specified by FC-4• I/O channels mapped by FC-PH

– Small Computer System Interfaces (SCSI) supports.• High capacity devices like disks.• High data rate devices like graphics and video equipment.

– High Performance Parallel Interface (HIPPI).• Used for mainframe/supercomputer environments.

– Network protocols mapped by FC-PH.• IEEE802 Mac frames to FC frames.• ATM• IP


Fiber Channel Transmission Media (Slide 1)

• Single mode fiber– 10Km at 800Mbps, 10Km at 400Mbps, 10Km at 200Mbps

• 50 um multimode fiber– 0.5Km at 800Mbps, 1Km at 400Mbps, 1500m at

200Mbps, 1500m at 100Mbps.

• 62.5 um multimode fiber– 175m at 800Mbps, 350m at 400Mbps, 1500m at 200Mbps,

1500m at 100Mbps.


Fiber Channel Transmission Media (Slide 2)

• Video coaxial cable– 25m at 800Mbps, 50m at 400Mbps, 75m at 200Mbps,

100m at 100Mbps.

• Miniature coaxial cable– 10m at 800Mbps, 15m at 400Mbps, 25m at 200Mbps, 35m

at 100Mbps.

• Shielded twisted pair– 50m at 200Mbps, 100m at 100Mbps.


Fiber Channel Topologies(Slide 1)

• Fabric or switched topology– An arbitrary topology requiring at least one switch to

interconnect a number of N_ports.

– The fabric may be a switched network of a number of switches, with or without end nodes.

– Each port in fabric has a unique address.

– Destination address of data from a node is examined by an edge switch and is appropriately forwarded to a node or another switch enroute to the node.


Fiber Channel Topologies(Slide 2)

• Point to point topology– Only two N_ports directly connected.

• Arbitrated loop topology– Connects up to 126 nodes in a loop.– Ports on loop must support N_port and F_port functionalities.– Ports are called NL_ports– Operation is roughly equal to token ring protocols.– Each port sees all frames and passes them along, keeping the

ones it needs and ignoring the ones not addressed to it.– A token access protocol is used to get loop access by a port


Wireless LANs Terminology

• Standards defined by IEEE 802.11 committee.• Basic services set:

– Contains stations executing the same MAC protocol.

– Stations compete to use the shared wireless medium.

• Access point– Allows a basic set to connect to a backbone distribution

system.

– Access point acts as a bridge.


Wireless LANs Terminology (Cont.)

• Distribution system– Connects two or more basic sets together.

– Typically it is a wired backbone LAN.

• Extended service set– Collection of BSSs connected by a distribution system.


Types of Wireless Stations

• Definitions specified in IEEE 802.11, based on mobility.

• Three types of stations are specified:– No transition

• Usually stationary.

• May be mobile in direct communication range of communicating stations within the BSS.

– BSS transition• Station moves from one BSS to another within the same ESS

– ESS transition• Station moves from one BSS in one ESS to a BSS in another ESS.


Wireless Physical Medium Specification

• 802.11 defines 3 physical media– Infrared

• 1Mbps using range 850nm to 950nm.

• 2Mbps using range 850nm to 950nm.

– Direct sequence spread spectrum• 7 channels, each with a data rate of 1Mbps or 2 Mbps can be used.

• Operates in 2.4GHz ISM band.

– Frequency hopping spread spectrum• Operates in 2.4GHz ISM band.

• Details are still being worked on.


Wireless Media Access Control (Slide 1)

• Two types of MAC algorithms were considered:– Distributed access protocols

• Decision to transmit was distributed over all nodes using a carrier sense mechanism.

• Makes more sense for the adhoc peer station networks.• Good for networks whose traffic is busy.

– Centralized access protocols• Access is managed by a central management station.• Makes more sense in wireless networks including base stations

attached to backbone networks.• Good for networks whose traffic is time sensitive and high priority.



• Distributed Foundation Wireless MAC (DAFWMAC) was the final product of 802.11 committee’s efforts to come up with a wireless MAC protocol.

• Allows distributed access control mechanisms.

• Allows central control of access as an optimal feature

• Contains 3 layers:– Physical layer

– Distrbuted coordination function (DCF)

– Point coordination function (PCF)



• Distributed coordination function– Uses CSMA algorithm.

– No collision detection function is used.

– A priority scheme based on a set of delays is used.

– A single delay is known as an interframe space (IFS)



• The rules of CSMA algorithm– Transmitting station senses the medium and may transmit

if it finds the medium to be idle for one IFS.

– In case the medium is found busy, transmitting station defers transmission and continues to sense the medium until current transmission ends.

– If the medium is found busy over another IFS, transmitting station backs of for a random period of time and tries again later using a binary exponential backoff algorithm.



• Three values of IFS are used:– SIFS

• Short IFS is used for all immediate response actions.

• Upon reception of a frame ack frames are sent after a SIFS delay.

– PIFS• Point coordination function IFS is used for central access control.

– DIFS• Distributed coordination function IFS is used as a minimum delay

for asynchronous frames contending for access.



• Point coordination function– Built on top of DCF.

– A central point coordinator issues polls in a round robin fashion to stations and offers each one the medium as required by it.

– The central point coordinator gains control of the channel through the CSMA mechanism described above.

– The central point coordinator has to relinquish control of the medium after it has controlled it to some pre set time.

© Copyright 1997, The University of New Mexico 4-1 LAN Technologies Ethernet Token Ring FDDI...

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Transcript of © Copyright 1997, The University of New Mexico 4-1 LAN Technologies Ethernet Token Ring FDDI...